Purpose: Extension workers' knowledge and competencies are fundamental to a successful practice. Most academic literature focuses on how to train extension workers, but neglects how complex their learning processes are. This article contributes to the topic by describing and analyzing extension workers' sources of knowledge and learning. Design/Methodology/approach: Twenty-four interviews and six focus groups were conducted with 56 extension agents working at the two most important Argentine extension institutions. Records were transcribed and categorized using Atlas.Ti software, and results were analyzed drawing on learning theories such as 'Theory of Action,' 'Experiential Learning' and 'Communities of Practice.' Findings: Scholars and institutions tend to neglect informal learning processes that take place spontaneously in the context of practice, such as learning from experience and from peers. Exchange among peers and graduate studies play a key role in connecting different learning sources and facilitating reflection on practice processes. Practical implications: Institutions could contribute to the development of extension workers' competencies by facilitating and institutionalizing informal learning, particularly horizontal exchange among peers, networking between practitioners from different units or territories, and mentorship. Theoretical implications: The different knowledge and learning sources interact, complement and generate synergies between them. These processes allow for the exchange and co-creation of knowledge. Critical reflection on practice contributes to the implementation of innovative extension strategies. Originality/Value: This article characterizes extension workers' sources of learning and knowledge and describes how they interact. Informal learning sources have usually been neglected by academic literature on the topic.